There's one question that I keep coming back to when thinking about game design: how do you tell the difference between a game that's fun to play, and one that's painful to stop playing? Subjectively, it can be hard to tell the difference, and objectively, players will buy and play one just as much as the other. However, there are clues that can help tell them apart. I've already talked about some of them in my post about the illusion of accomplishment, but today I would like to focus on unfinished stories.
The basic structure of a traditional storyline goes like this: Something happens that incites conflict in the protagonist's world, and she acts to resolve it. Eventually, she succeeds, and a new balance is reached. Here's a diagram of this structure:
Most stories are more complicated, and use a variation of this structure with some more nooks and crannies. However, the introductory problem is always resolved by the end. There is an alternative, though. If you don't mind compromising your story in order to get your audience to buy your next work, you can just skip the ending.
Ending on a cliffhanger
The word 'cliffhanger' evokes old films that literally ended with the protagonist hanging from a cliff, forcing the audience to choose between buying another ticket to see what happens, or spending the rest of their lives with a half-finished story in their heads. Here's a diagram of the cliffhanger structure:
This is becoming increasingly popular in-games, with some extreme examples including Assassin's Creed, Halo 2, Half-Life 2, Prince of Persia, and God of War 2. Often the game ends with the villain saying, "Haha! You may think you've won the battle, but I will still win the war!" These endings can leave gamers feeling unsatisfied, or even cheated. For me, the ending of Assassin's Creed stands out as the worst gaming experience I ever had. So why do game developers keep using cliffhanger endings? Because it makes the player uncomfortable if he doesn't buy the sequel, and it allows the developers to reuse the same story and enemies in the next game.
A more advanced version of the cliffhanger structure has been used by 'soap opera' shows for decades. They always have several story threads running at the same time, and whenever one is resolved, there are several other threads that are still up in the air. Recently this technique has spread to action stories as well, resulting in addictive shows like 24, Lost, and Heroes. These stories sacrifice some coherency and depth in order to cycle plot threads as fast as possible -- maximizing the amount of "stuff happening" at any given time. Here is an example of an interleaved story:
This structure also leaves a cliffhanger ending, but disguises it by resolving an unrelated story thread. This structure is used in a number of games, including Metal Gear Solid, Final Fantasy, and Mass Effect. An advantage of the interleaved story structure is that it repeats indefinitely, making it easy to add and remove story segments to change the story's length. However, this is also a disadvantage; it encourages adding more and more complexity to the plot, and there's a limit to how much plot complexity you can add before the story just stops making sense.
Stories need to end
The ending of a story really reveals its meaning and intent, and a cliffhanger shows the audience that the intent is to sell a sequel. Many games, including Portal, Sands of Time, and Braid are remembered fondly in large part due to the attention paid to their endings, and there's nothing stopping the developers from creating sequels anyway.
Selling games with unfinished stories may be an effective technique to trick gamers into paying more money, but to me it seems a lot like selling someone poisoned food, and then selling them the antidote. Sure, you might make more money, but is that really how you want to do it?